Thursday, July 22, 2010

The sperm whale fossil record had still not revealed all its secrets – until now! With the exception of a few isolated, large teeth, only animals significantly smaller than modern sperm whales have been discovered. In November 2008, during excavations organised by an international team (Muséum National d'Histoire Naturelle / Centre National de la Recherche Scientifique (CNRS); Museo de Historia Natural, Lima; Università di Pisa, Pisa; Natuurhistorisch Museum, Rotterdam; Muséum des Sciences Naturelles, Bruxelles) in the coastal desert of the Ica region (southern Peru), the remains of a very large fossil sperm whale were unearthed in Miocene beds (12-13 million years ago) at the Cerro Colorado site. The skull, lower jaw and teeth of this giant predator were recovered and prepared and have formed the object of a joint study, the results of which will be published in this week's edition of the journal Nature.

The morphology of the newly discovered sperm whale differs considerably from that of the modern sperm whale. Despite its similar size, with a skull which is 3 metres long and an estimated total body length of between 13.5 and 17.5 metres, this animal, which has been named Leviathan melvillei in honour of Herman Melville and his famous novel “Moby Dick”, has extremely strong teeth (on the lower jaw as well as the upper jaw). In fact, the largest teeth are more than 36cm long and have a diameter which can reach 12cm!

Given its size and the strength of its jaws and teeth, Leviathan was probably a superpredator, capable of feeding on large prey by trapping it in its powerful jaws and using its impressive teeth to kill it. Furthermore, Leviathan was discovered in geological layers dating from an epoch (end of the middle Miocene) during which the diversity of mysticetes (baleen whales) considerably increased. Some species of whale also reached significant sizes (around ten metres). Therefore, scientists have put forward the hypothesis that this large predator fed on baleen whales, as a large number of skeletons have been found at the Cerro Colorado site, where Leviathan was discovered.

It is also interesting to note that the teeth of another very large marine predator, the giant fossil shark Carcharocles megalodon, have also been discovered in large numbers at the Cerro Colorado site. So there could have been two superpredators fighting over prey with a very high nutritional value - baleen whales.

While the descendants of squid-hunting sperm whales have survived into our times, as modern sperm whales (Physeter), Leviathan and other predatory sperm whales disappeared at the end of the Miocene or Pliocene epochs. Scientists still do not know why, but decreased diversity in their prey, baleen whales, at the end of the Miocene period, as well as climate change, may have played a role in their extinction. During the Pliocene, another group of toothed cetaceans would specialize in hunting large marine mammals; this group includes modern killer whales, Orcinus orca. Although significantly smaller than Leviathan (total size of less than 9 metres), through working together, killer whales are able to kill and consume large cetaceans (rorquals, humpback whales, grey whales, etc.).

This giant sperm whale's skull is on display at the Lima Natural History Museum (Peru).